cogoman wrote: > > *It seemed counter-intuitive to me, but from reading stepper motor > controller data sheets I've found that if you short out a winding, the > current takes longer to decay than if you put a resistor across it, and > make it pump energy into a higher voltage. I don't know how this applies > to the changing fields in a spinning universal motor, though.* > This is the difference between inductance and back-EMF. If you short a perfect inductor, the current will recycle forever. This is how superconducting magnets work, the current in the coil is ramped up, then shorted, and the power supply is turned off. If you add resistance to the circuit, the current MUST decay, as the energy stored as magnetic field is being dissipated in the resistor.
But, in a permanent magnet motor, the current can increase dangerously if there is a large source of mechanical energy, so it is best to use a resistor to limit current. In a universal motor, there are no permanent magnets that could be damaged. If these motors are started from dead stop across a 120 V AC line, then shorting them is not likely to produce higher currents than that. Jon ------------------------------------------------------------------------------ Simplify data backup and recovery for your virtual environment with vRanger. Installation's a snap, and flexible recovery options mean your data is safe, secure and there when you need it. Data protection magic? Nope - It's vRanger. Get your free trial download today. http://p.sf.net/sfu/quest-sfdev2dev _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
